Insulating tank for cold boiling liquids

ABSTRACT

789,755. Storing liquefied gases. CONSTOCK LIQUID METHANE CORPORATION. July 15, 1955 [April 6, 1955], No. 20628/55. Class 8(2). A liquefied gas storage vessel at substantially atmospheric pressure comprises an outer metal casing 1, Fig. 3, lined with means whereby a cushion or blanket of vapour is interposed between the liquid 12 and the casing wall. A plurality of spaced superposed louvres 9 of heat insulating material secured round the casing wall have skirts 10 which trap vapour in pockets 11. The louvres 9 may be lined with balsa wood or embedded in such blocks 16 as shown in Fig. 5. Alternatively the louvres are dispensed with and the casing lined with blocks of balsa wood glued together and having the grain running horizontally with the casing wall. The liquid seeps into the wood assisted by capillary attraction and is there vaporized before reaching the casing wall to provide the blanket of vapour.

Nova 11, 1958 S- BECKWITH INSULATING TANK FOR COLD BOILING LIQUIDS Filed April 6 1955 5 Sheets-Sheet 1 INVENTOR,

Serlz'nj by, Parker 8 Carter,

ATTORNEYS s. BECKWITH INSULATING TANK FOR COLD BOILING LIQUID S s Sheets-Sheet 2 I Nov. 11, 1958 Fi 1ed Ap?i1 6 1955 Nov. 11, 1958 s. BECKWITH I INSULATING TANK FOR cow BOILING LIQUIDS Filed April 6, 1955 3 Sheets-Sheet 3 Emm r n mw in 05 1.0% n a a p 1m a 3321? 4 :3 Z10 ftjl 4 A nited States INSULATING TANK FOR coL-D noILING LIQUIDS Application April 6, 1955, Serial No; 499,570 6 Claims. (Cl. 220-65;

My invention relates to improvements in method and apparatus for storing and shipping cold boiling liquids at low pressure and ambient temperature.

By cold boiling I mean liquids which have an active liquid boiling point, at atmospheric pressure, of less than 60 F. and equilibrium vapor pressure of one atmosphere at the atmospheric boiling temperature;

By low pressure I mean such pressure, approximating atmospheric, that in designing and constructing a tank to contain the liquid, the difference between the gas pressure and the atmospheric pressure outside of the tank may be disregarded.

By' ambient temperature I mean the temperature of the ambient air to which the tank containing theliquid is exposed in storage or transit. This temperature depending on season and geographic conditions might well vary between somewhat below F. and somewhat above 110 F.

My invention'is' especially well adapted to storage andtransportation of such a cold boiling liquid as natural gas, or its components, the boiling point of which, at atmospheric pressure is in the order of 258 F. The invention, however, is also applicable to the storage and transport of any liquid, which at the atmospheric pressure and ambient temperature prevailing at the time when and the place where the liquid is located needs insulation to protect it against excessive boiling or evaporation.

I propose to enclose the liquid in a tank which may be fixed in place or may be mounted on a boat, railroad car or other vehicle for shipment. The tank will include a wall, shell or barrier, impervious to liquid or gas and will have a relatively thick insulating lining, pervious to liquid and gas and in direct contact with the liquid in the tank. The shell and the lining will together form a' self-supporting tankstructure wherein either or both furnishes the structural strength.

The shell, especially in the larger sizes, will be preferably of heat conducting metal. Under some circumstances, the shell may be of plastic or other non-metallic material, but it must in any event, be impervious to gas and liquid. The lining must completely mask the shell and must be of a material which does not appreciably lose its insulating value and structural strength in contact with the cold liquid and gas.

The lining limits the rate of heat inflow from the shell, warmed as the latter is by the ambient air, to the cold liquid mass in the tank and completely prevents contact of the cold liquid with the shell.

The shell and lining of the empty tank will usually be at ambient temperature. When the tank is filled with the cold liquid the lining will be cooled but the shell will not be cooled and the lining will shrink. To prevent rupture of the porous lining, which would permit the liquid to contact the shell, the lining will be installed in the shell, under pressure sufficient to compress it at least as much as and preferably more than the shrinkage resulting from such cooling.

2,859,895 Patented Nov. 11, 8

ice

When the tank is filled, the level of the cold liquid will usually be somewhat below the top of the tank and the space above the liquid will be filled by the gas'or Vapor evaporated therefrom. The'li'quid,'in direct-contact with the pervious lining, will migrate'therethrough toward the shell in capillary filamentsi T he force: causing this migration will be in part capillary attraction'and' in part the hydraulic pressure ofthe-liquid above" thepoi'n't of penetration at each capillary. Capillary attraction will be generally uniform but the hydraulic pressure decreases as the distance between the point of penetration and the surface of the liquid decreases.

Assuming for example that ambient temperature is '60? F. and the temperature of the gas is' 258 F. theouter boundary of the insulating-zone where it contacts the shell will be somewhat below approximately 58 F. and the inner boundary of the zone in contact with the liquid mass will be at 258 F. The temperature gradient will be 316 F. and since the insulation resists heat flow generally uniformly, the temperature in the lining will drop steadily from the shell toward the liquid.

As the capillary filaments migrate outwardly, being. of relatively large surface area in relation to their volume and cross section, they will be exposed to progressively increasing heat so that before they can penetrate the insulating zone deep enough to contact the shell they will be vaporized and the resultant vapor if it. continues its travel toward the shell will be further heated so as to contact the shell at a temperature much higher thanlthe boiling temperature. 7

Assuming that the gas is able to escape freely from the insulation a position of equilibrium will be reached for the prevalent ambient temperature such that the liquid filaments continue their migration to. the limit of'their excursions where they are finally vaporized. Because hydraulic pressure decreases upwardly, the power urging penetration decreases upwardly and so distance of travel of the filaments outwardly will decrease as the level of the liquid is approached.

On the other hand, if the escape of the gas boiled off from the capillary filaments is resisted or perhaps even prevented, a pressure will be built up in the insulation which resists the outward migration of the filaments and may even force them back toward the liquid mass.

The pressure of the gas will under most circumstances be sufiicient to expel the liquid from the pervious lining so that boiling will take place along the inner boundary of the lining where it is contacted by the liquid in which case the gas will pass up for discharge to the gas pocket above the liquid level. If the gas migrates upwardly through the lining, it will finally reach an area where it-can pass outwardly through the lining to enter the gas pocket or gas dome above the liquid level. Perhaps both paths may be followed by the gas. In either event, so long as heat sufiicient to cause boiling reaches the liquid mass, boiling will take place without substantial change in the temperature of the mass just as steam escapes from boiling water on the stove without substantial change in temperature of the water mass. In this case, since I propose storage and ship ment of the gas at atmospheric pressure, the gas will be allowed to escape from the gas pocket in the tank from above the liquid level at such rate as will-maintain the desired pressure.

It is desirable that the migration of gas upwardly through the lining be resisted so that a pressure develops in the lining which will positively prevent outward movement of the capillary filaments into actual contact with the tank wall.

I propose for example, one construction of tank lining 3 comprising a wall made of a multiplicity of separate blocks or strips or panels of a light, permeable, preferably straight grained, natural or synthetic wood-like material having a high insulating factor. The separate elements comprising the Wall will be assembled with the grain horizontal and parallel to the tank wall so that a minimum of end grain will be exposed to the liquid. Such material will normally be less pervious to gas and liquid across the grain than parallel with it. So the arrangement I propose offers an adequate resistance -t o penetration of'liquid toward the shell and an adequat'e resistance to escape of gas upwardly along the shell.

i It will be preferable usually to cement or glue the individual pieces in place in the wall and the glue planes willoffer additional resistance to capillary penetration of liquid and to discharge of gas through the body of insulation. If desired, these glue planes may be associated with additional barrier means to limit migration of liquid and gas but such glue planes or any such barrier means will be so staggered in the assembly that no direct unobstructed passageway for heat, gas or liquid will be left in the insulating wall.

I have found that balsa wood is today from every point of view an ideal material for this purpose but other materials may under some circumstances be used 'with entire satisfaction.

The pores in the lining provide a multiplicity of 7 minute gas pockets which enclose the gas between the 'shell and the liquid body and support by the pressure of the gas therein the hydraulic pressure and capillary attraction tending to urge the filaments toward the shell.

' The same principle is involved when in the absence of such a porous lining steps are taken to interpose beltween the body of cold boiling liquid and the shell or tank wall a multiplicity of larger gas pockets wherein the gas evaporated from the liquid may be trapped to build up a pressure sufiicient to prevent contact of the liquid with the shell. One way in which this can be done is by 'providing around the inner periphery of the tank wall a plurality of louvres inwardly and downwardly inclined, each louvre overlapping the one below it so that a series of gas pockets are provided between the louvres wherein the gas evaporated from the liquid will build up a pressure and limit the penetration of the liquid into the pocket in much the same manner as the air lock used in under water construction builds up a pressure to keep the water out of the area in which the men are working.

Such louvres can be of material of limited conductivity and because the pressures are balanced between the gas and the liquid, those louvres can be light and even flexi- 1ble. They may, for instance, be made of any one of the plastic compositions which retain sufficient strength and impermeability for this purpose under the conditions of low temperature involved.

Under some circumstances such louvres may be associated with a porous material such as the lining above referred to, either facing the louvres or completely filling the space between them. In the latter case, the louvres will furnish a positive resistance to escape of gas upwardly through the lining along the face of the tank.

My invention is illustrated more or less diagrammatically in the accompanying drawings, wherein Figure 1 is a perspective in part section;

Figure 2 is a section on an enlarged scale through the insulation;

Figure 3 is a section on an enlarged scale through a portion of a tank wall showing a modified form of insulation using louvres to define gas pockets;

Figure 4 is a section similar to Figure 3 showing the louvres associated with a porous material;

Figure 5 is a similar section to Figure 4 showing the pockets between the louvres filled with the porous material;

. Figure 6 is a further modification where the louvres are in part lined with porous material and in part freely project from the porous material into the body of the tank.

Like parts are indicated by like characters throughout the specification and drawings.

1 is a tank shell. It is preferably of suitable structural steel and is both liquid and gas impervious. 2 indicates generally an insulating lining for the side wall and 3 an insulating lining for the floor. The roof lining is to all intents and purposes the same as the floor lining and in the interest of clarity is not illustrated.

The lining is made up of a plurality of slabs or sections or elements 4 which may be cemented together along vertical horizontal glue planes 5 and 6 respectively. The planes are staggered'so that there is no uninterrupted glue plane extending from the liquid mass to the outer shell or barrier. 7 indicates the grain or pores of the lining element. If, as may frequently be the case, it will be of natural straight grain wood, the pores or grain are arranged generally horizontal and parallel with the shell.

In the modified form shown in Figure 3, a plurality of louvres have vertical flanges 8 cemented or otherwise attached to the inner periphery of the shell 1. Intermediate portions 9 are downwardly and inwardly inclined and vertical terminal skirts 10 extend downwardly parallel with the wall. The skirts 10 extend downwardly a distance substantially below the upper extremity of each lower louvre so that a plurality of gas pockets 11 are defined between the shell 1, the downwardly and inwardly inclined portions of two of the louvres, the skirt of one and the liquid mass which is indicated at 12. As the tank is filled with the liquid 12, as the liquid rises, whenever the level of the liquid reaches the lower extremity of one of the skirts 10, the pocket above the liquid level between that skirt, the intermediate portion of its louvre. and the wall is closed and the heat entering through the shell 1 causes vaporization and gas fills the pocket 11. There is no escape for the gas and so the pocket remains a gas pocket without penetration of the liquid into the pocket above the point at which the pressure of the gas overcomes the hydraulic pressure of the liquid. Any further evaporation of the liquid increasing the pressure in the pocket 11 above that necessary to overcome the hydraulic pressure will merely result in some of the liquid bubbling out as indicated at 13 for travel upwardly along the insulating area for discharge to the gas pocket 14 above the level of the liquid.

These louvres will preferably be of some material which has a very low degree of heat conductivity, preferably some of the plastics such as Teflon or Kel-F. These louvres extend clear around the tank and so each pair of louvres define an annular pocket extending clear around the tank. Since there is gas pressure in each of the pockets below the liquid level, the louvres can be of relatively thin, light material. Under some circumstances they might be of very thin metal, such as some of the nickel alloys, which would not be deteriorated by or lose its strength as a result of contact with the cold liquid and have a very high thermal resistance. A very thin metal foil would have a very low conductivity and under some circumstances there would be no objection to that although ordinarily a plastic sheet is preferable.

In the modified form shown in Figure 4, the louvres are formed in part of pervious and in part of impervious material similar to the lining material disclosed in Figures l and 2. In this case, the impervious element is shown below the porous material to insure that there will be no escape of the gas through the pervious material to the next higher gas pocket.

In the modified form shown in Figure 5, the louvres are embedded in the porous lining. In this case, each annular gas pocket defined by the louvre is closed against the escape of upward movement of the gas,

Referring to Figure 4, it will be noted that a relatively thin sheet of porous material 15, together with the plastic or metal louvre 9 defines the upper boundary of each of the annular pockets. In Figure 5, a plurality of insulating porous pieces 16 are arranged about the periphery of the tank being interrupted by the impervious louvres 9. In Figure 6, the louvres 9 as indicated extendinwardly beyond the insulating porous material 16.

The use andoperation of my invention are as follows:

As the tank with its pervious lining and impervious outer shell is about to be first filled with the cold liquid, the shell and lining will both be at ambient temperature far .above the boiling point of the cold boiling liquid and will be filled with air. Steps must first be taken to expel the air. This may well be done by filling the tank with carbon dioxide or other inert gas which mixed with the vapor from the liquefied gas will not form an explosive mixture.

.Thefillin'g of the tank with such an inert gas which should be heavier than air will to some extent purge the pervious lining itself from air but in any event there will. be so little air left in the lining that the possibility of forming an explosive mixture can be disregarded.

When the cold liquid is thereafter placed in the tank, it will boil from the heat in the lining and so there will always be the liquid in gaseous or vaporous phase above the surface of the liquid as the tank is filled. The cold liquid will chill the porous lining of Figures 1 and 2 tending to cause it to shrink but since the lining is already under a compression at ambient temperature, the chilling will merely lower that compression pressure and since the compression pressure will shrink or compress the lining more than it will shrink from the cold, a continuous permeable insulating barrier will remain between the cold liquid and the tank wall.

The liquid will wet the lining and since the lining is pervious to the liquid, the liquid will tend to enter the lining along capillary pores being drawn in by capillary attraction. It will also be forced in by hydraulic pressure of the liquid colu'mn above each point of penetration so there can be a gradual migration of liquid in capillary filaments into the lining.

Since there is a temperature gradient between the ambient temperature of the shell and the cold boiling liquid, that gradient will result at some point between theinner and outer surfaces of the. lining in causing the heat applied to the various capillary filaments to boil and evaporate the liquid. When the liquid evaporates, the gas expands, at or about atmospheric pressure, being many hundreds of times in volume greater than is the liquid and a pressure will be built up. The gas under this pressure will seek an outlet. Two possible outlets are open, one back into the tank in the reverse direction to the path of the capillary filaments, the other upwardly to a point above the level of the liquid. Because the porous or pervious lining is filled with a multiplicity of very fine capillary passages, it is through these narrow passages that the gas must travel after vaporization.

The distance between the liquid mass and the shell in a direction perpendicular to the shell is much less than the distance from the point of capillary penetration to a discharge in the gas pocket above the liquid everywhere except just at the top of the tank. The longer the capillary passage to be traveled by the gas, the greater the resistance. Therefore, especially since in the case of wood where the grain is parallel with the shell and horizontally disposed, the resistance to gas movement or migration upwardly from point of vaporization for discharge to the gas pocket is at a maximum.

On the other hand, the resistance to travel of gas back from the shell to the mass of liquid comes in part from resistance to flow through the capillary in part from the capillary attraction between the walls of the capillaries and the liquid and in part from the hydraulic pres- 6 sure head. The balance between the resistance to movement of gas upwardly toward the gas pocket on the one hand and horizontally back toward the liquid on the other hand depends on the relationship between the" distance traveled, the hydraulic pressure and the capillary attraction. I 7

Under ordinary circumstances the gas generated in various parts of the lining will some of it return to the zone of contact of the liquid mass and the lining and some of it will be dischargedto the gas pocket. The relationship between these discharges is of small moment so long as the evaporation of the liquid, the generation of the gas and the pressure generated thereby and the time available for such evaporation is such that no liquid ever touches the wall or shell of the tank.

Under the circumstance that the pressure is great enough to force the liquid back'out of the lining boiling may take place in the boundary zone between the liquid mass and the lining but in view of the heat gradient, it may frequently happen that the filaments will be forced out by the pressure and the pressure will fall somewhat and the filaments again penetrate and again be expelled.

it makes no difference whether the filaments are forced back or merely arrested provided only that all the filaments or liquid body are vaporized before any of the liquid touches the impervious shell.

Thus the liquid is always enclosed in a blanket or cushion or wall of the gas, the gas wall or cushion being localized in the insulating lining and the boundary zone between the liquid and the gas cushion being generally at the surface of the insulation but always continuous to the extent that the gas cushion is' interposed between the liquid and the shell throughout their entire opposed areas.

In the modified form disclosed in Figure 3, capillary resistance to movement of gas and liquid is not present. Here it is only the pressure of the gas in the individual gas pockets which prevents contact of the liquid and the shell but since the gas itself is an exceedingly poor conductor and since each pocket is gas tight above the level of the liquid in the pocket, there can be no escape of the gas and again a continuous or substantially continuous cushion is formed between the liquid mass and the shell.

The cushion serves as an insulator. It is sufficiently flexible to compensate for variations in pressures and temperatures and remains continuously, as long as there is cold boiling liquid in the tank, as a'barrier between the liquid and the tank wall whereby the tank wall is never allowed to be contacted by and so brought down to the temperature of the cold boiling liquid. This cushion whether resulting from the louvre structure of Figure 3 or the louvre structure in combination with porous or pervious material as in Figures 4, 5 and 6 or from the pervious porous material of Figures 1 and 2 is always a living, flowing, continuously generated cushion which controls and always remains interposed between the solid mass of liquid and the shell.

If the pervious lining is used, it is wetted by the liquid, the liquid is vaporized and the living, continuous cushion of gas is localized and held between the liquid and the shell by the pervious lining. On the other hand, if the louvres or some similar mechanism are used to define a multiplicity of relatively large gas pockets, those gas pockets together define the continuous, living, flexible cushion interposed at all times between the liquid and the shell.

The material between the capillary pores or the material of the glue planes or the material of the louvres being themselves not good conductors and being of relatively small cross sectional area in proportion to the surface of the shell masked by the cushion from contact with the liquid do not permit any excess heat penetration. Some heat penetration to the liquid mass will always take place, under most circumstances is desirable, can be predicted and controlled. The important thing is that the heat penetration is always from the shell through the gas cushion to the liquid but the liquid itself, as liquid, never contacts the shell and the gas which does contact the shell is always much warmer than the liquid and of such low specific heat that the temperature of the shell is not raised appreciably above ambient temperature.

' This gas cushion will increase in temperature outwardly toward the shell because of the temperature gradient and gas in contact with the shell will be at a relatively high temperature even though it may be below the temperature of the shell but the shell is always exposed to gas, not to the liquid, so the shell temperature seldom if ever falls below the point at which condensation ofambient air moisture will take place and never below the point at which any deleterious temperature etfect will be felt by the shell.

Where I have used the word wall or shell that word is intended to means the impervious outer layer through which neither gas nor liquid can penetrate whether it is on the side, top or bottom of the tank or storage reservoir defined thereby.

The particular shape of the louvre is not important. All that is required is that the lower inner edge of the louvre be far enough below the zone of contact of the louvre with the shell that the level of the liquid in the pocket defined by each two adjacent louvres be such that the liquid never contacts the shell and always contacts the lower louvre below its area of contact with the shell. Since the base of the louvre in contact with the shell is out of line with the inner portion of the louvre shrinkage of the part of the louvre chilled far below the temperatureof the shell may cause distortion but since that part of the louvre in contact with the shell is at shell temperature a gas tight seal will continue to exist even if a part of the louvre should draw away from the shell.

The pockets are defined by the impervious louvres, may be filled with porous or pervious material. Since the impervious louvre limits and controls gas escape the filler for the pockets may, if desired, not be under initial compression but even if clearance develops about or within the louvre filler material the presence of the filler will still inhibit convection currents in the gas in the pocket.

While I have referred to the gas pockets which form the insulating cushion to insulate the liquid mass it will be understood that while there is a possibility that some air may also be entrapped the air will be such a small proportion of the contents of the gas pocket that the air may be disregarded and by the term cushion or pocket of the vapor of the liquid I mean a cushion which is predominately composed of the vapor of the cold boiling liquid in the tank.

It is preferable that evaporation of the liquid take place along the zone of contact between the cold liquid and the insulation exposed to and in direct contact with the liquid. When that situation prevails, the vapor cushion will be located between the liquid and the insulation and as vaporization continues the vapor will migrate upwardly along the area of contact toward the vapor dome above the liquid.

With respect to the pervious insulation, this preferred threshold is reached when the escape of vapor upwardly through the porous insulation is at a lower rate than the rate of evaporation at the surface of the insulation.

Under some circumstances it may happen that gas generated by evaporation within the pervious lining may migrate upwardly and return to the area between the lining and the liquid at a point below the liquid level depending upon possible differences in the resistance to gas migration in various parts of the lining.

I claim:

1. A tank for storage at low pressure of cold boiling liquids which includes a shell impervious to liquid and gas, a porous, liquid and gas permeable insulating lining interposed as a lining between the shell and the liquid and directly in contact with the liquid, the lining being more permeable along horizontal lines parallel with the shell than it is along horizontal lines perpendicular there to so as to provide greater resistance to the flow of liquid and vapors endwise through the lining from the liquid to the shell than to the flow of liquid and vapors through the lining parallel to the shell.

2. Atank for storage at low pressure of cold boiling liquids which includes a shell impervious to liquid and gas, a liquid and gas permeable insulating lining interposed as a lining between the shell and the liquid and directly in contact with the liquid, the lining being more permeable along horizontal lines parallel with the shell than it is along vertical lines parallel with the shell so as to provide greater resistance to flow of liquid and gases through the lining in a vertical direction as compared to the direction of flow of liquid and gases through the lining in a horizontal direction.

3. A tank for storage at low pressure of cold boiling liquids which includes a shell impervious to liquid and gas, a liquid and gas permeable insulating lining interposed as a lining between the shell and the liquid and directly in contact with the liquid, the lining being more permeable along horizontal lines parallel with the shell than it is along horizontal lines perpendicular to and along vertical lines parallel with the shell so as to provide greater resistance to flow of liquid and gases through the lining in a vertical direction as compared to the direction of flow of liquid and gases through the lining in a horizontal direction.

4. A tank for storage at low pressure of cold boiling liquids, including a shell impervious to liquid and gas, a liquid and gas permeable insulating lining therefor, exposed to direct contact with the liquid, built up of a multiplicity of substantially straight grained wood-like sections, cemented in place to completely mask the shell, the grain of the sections being generally horizontal and parallel with the adjacent shell wall so as to provide greater resistance to flow of vapors and liquids endwise through the lining from the liquid to the shell as compared to the flow of liquids and vapors through the lining parallel to the shell.

5. A tank for storage of cold boiling liquids including a shell impervious to gas and liquid, a plurality of horizontally disposed louvres extending inwardly and downwardly about the vertical inner boundary of the shell, arranged one above the other, the inner lower periphery of each louvre terminating substantially below the upper, outer periphery of the louvre below it, each louvre being in impervious contact with the inner wall of the shell, being itself impervious to gas and liquid and being composed of such material so dimensioned as to have low heat conductivity.

6. A tank for storage of cold boiling liquids including a shell impervious to gas and liquid, a plurality of horizontally disposed louvres extending inwardly and downwardly about the vertical inner boundary of the shell, arranged one above the other, the inner lower periphery of each louvre terminating substantially below the upper, outer periphery of the louvre below it, each louvre being in impervious contact with the inner wall of the shell, being itself impervious to gas and liquid and being made of material having low heat conductivity, the series of louvres defining a plurality of annular gas tight gas pockets closed at their lower boundary by the liquid in the tank.

References Cited in the file of this patent UNITED STATES PATENTS 644,259 Ostergren Feb. 27, 1900 2,148,109 Dana et al Feb. 21, 1939 2,663,626 Spangler Dec. 22, 1953 2,770,951 Morrison Nov. 20, 1956 

